1. Field of the invention.
The present invention relates to an apparatus for checking diametral dimensions of a mechanical piece, with support means, two reciprocally movable feelers for contacting the piece, transducer means for providing a signal responsive to the mutual distance of the feelers, transmission means located between the feelers and the transducer means and protection means for preventing the entry of foreign matter, these protection means including a substantially tubular element coupled to the feelers.
The invention also relates to an apparatus for checking inside diameters of a mechanical piece, comprising support means, two arms movable with respect to the support means and defining free ends, two reciprocally movable feelers cooperating with the free ends of the arms, transducer means for providing a signal depending on the mutual distance of the feelers, and protection means, for preventing the entry of foreign matter, including a resilient substantially tubular element.
Further, the invention relates to an apparatus for checking inside diameters of a mechanical piece, comprising support means, at least two reciprocally movable feelers for contacting the piece, transducer means, for providing a signal depending on the mutual distance of the feelers, with at least two mutually movable elements, protection means, for preventing the entry of foreign matter, with a resilient tubular element and transmission means arranged between the feelers and the transducer means and rigidly connecting said movable elements to the feelers.
2. Description of the prior art.
There are known plug gauges like, for example, the manually applied plug gauge disclosed in U.S. Pat. No. US-A-4,329,782, including an arm-set consisting of a pair of arms connected to a support and movable both mutually and with respect to the support, feelers coupled to the ends of the arms, and a differential transformer position transducer having windings connected to an arm and a core connected to the other arm, for detecting mutual displacements between the arms.
This gauge also comprises a nosepiece fixed to the support, that partially houses the ends of the arms carrying the feelers for protecting the armset from accidental impacts and centering the gauge with respect to the hole to be checked. The nosepiece defines two diametrically opposite holes for the passage of the feelers. These holes must be sufficiently large to enable the feelers to move freely therein and so must allow a certain amount of clearance between each feeler and the edge of the corresponding hole. Thus, through the resulting openings foreign matter may possibly enter inside the gauge and interfere with the arm-set, so causing malfunctions.
This hazard is particularly high in plug gauges installed on "transfer" lines or "transfer" machines: these plug gauges automatically check the dimensions of holes during intermediate phases of relevant machining cycles, consequently in an environment affected by coolants, chips and other residual products of machining. The entering and accumulating of similar foreign matter inside the nosepiece, into contact with the armset, can hamper the feelers from moving freely and may damage or interfere with the parts of the transducer. Consequently, it is necessary to frequently accomplish expensive cleaning operations involving removal of the nosepiece and stopping of the machining, in addition to the risk of damaging the armset during these operations.
Two different methods for facing this problem are followed in the known gauges applied to transfer machines, or in any case in those environments that are characterized by the presence of foreign matter. A first method is based on the discharge of foreign matter in order to prevent the buildup within the gauge. To this purpose there are foreseen additional openings through the nosepiece and there is used compressed air: this type of solution, apart from being particularly expensive for the provision of a specific pneumatic system, may induce the armset or the transducer to malfunction due to perturbations caused by the compressed air, especially if the flow is not constant and continuous.
The second method aims at preventing any foreign matter from reaching the armset.
A plug gauge operating according to this method is described in Italian patent No. IT-B-1145503, apart from having a gasket between the movable arms for protecting the electrical transducer from the possibility of liquids and/or dust entering inside, has another flexible gasket, of a substantially tubular shape and of variable thickness, with two ends resiliently fastened to a cylindrical body of the plug and two protruding annular portions coupled to the feelers. The inside of the plug body is sealed by means of the second gasket, the plug body itself and other elements. In order to protect the second gasket, that is obviously made of a soft material, not very resistant to impacts like, for example, those of hot chips, there is foreseen an external mechanical protection obtained by a stiff nosepiece coupled to the cylindrical body of the plug and provided with openings for the passage of the feelers, featuring limited clearance for preventing the entry of large chips. Furthermore, compressed air is made to flow out of the plug, in particular through an air space between the plug body and the nosepiece and through the openings of the nosepiece, in order to further hamper the entry of chips and other foreign matter. This known plug gauge, apart from being expensive, is considerably bulky, and therefore unsuitable for checking small diameter holes (for example smaller than 8 mm).
In spite of the use of supplementary protections, the risk of breaking the second gasket, even if considerably diminished, is not completely eliminated.
There are also disclosed in U.S. Pat. Nos. US-A-3,958,337 and US-A-3,958,338 plug gauges with arm-sets comprising metal laminae shaped in various ways (more specifically, M-shaped and U-shaped), to which the feelers are fixed. The laminae, at least partially, replace the movable arms and define, at various accuracy levels, the directions of displacement of the feelers. In these known gauges, particularly suitable for manually checking small diameter holes, the principal function of the laminae is not that of a mechanical protection.